ATMOSPHERE (Gk. etr,a6s, a-tmos, vapor + aq5aipa, sphaira, sphere, globe). The name ap plied to the mixture of gases and vapors sur rounding any climate or sun, but especially the envelope of our globe. Spectrum analysis tells us much about the constituent gases and vapors, and even the relative motion of these in the at mospheres of the earth, the sun, and the distant stars; we know in general that nearly all of the chemical elements that occur in those are also present either on our earth or in its own atmos phere. The principal constituents of the earth's atmosphere are oxygen and nitrogen, in the ratio of 21 to 79 respectively. The next most impor tant constituent is aqueous vapor, the proportion of which varies from 5 per cent. to nothing, de pending upon the temperature and location of the sample of air that is analyzed. The other gases that are most commonly found are car bonic acid gas, whose proportion varies slightly, hut may he considered as being on an average 0.04 per cent., and to this extent may be consid ered as a normal constituent; when this gas is in excess (as in many dwellings where the com bustion of fuel or illuminating gas is not prop erly guarded, or when persons are gathered to gether without proper provision for ventilation) it is to be considered as an undesirable impurity. Traces of ammonia, hydro-carbons, and possibly ozone, are frequently encountered. The most in teresting recent additions to our knowledge have resulted from our ability to cool and condense the air into a liquid form. By the fractional dis tillation of this liquid air, Rayleigh, Ramsay, Dewar, and others have shown the presence of gases whose densities, calling that of hydrogen 1 and oxygen 16, arc as follows: Helium, 1.98; neon, 9.97; argon, 19.96; krypton, 40.88, and xenon, 64. These rare gases are exceedingly vola tile, and their presence is held to indicate not that they belong to the earth's atmosphere, but that they are simply diffused through the whole interplanetary space, and are not held down to the earth's surface to any great extent by the attraction of gravitation. On the other hand, a larger body like our sun may have an attractive power sufficient to accumulate a larger propor tion of some of these gases in its atmosphere.
The chemical and physical properties of the earth's atmosphere are of vital importance to mankind. In a general way it is ordinarily stated that the stratified portion of the earth's crust, including the fossils, the coal-beds, the oils, the gases, and all hydro-carbon compounds, as well as all the water contained in the rocks, the oceans, and lakes, represents an immense vol ume of oxygen, nitrogen, hydrogen, carbonic acid gas, etc., that has been abstracted from the
primitive atmosphere of the earth, which must therefore have at one time been very much greater than at present. It is supposed that at the present time the formation of new com pounds and the disintegration of others on the earth's surface is going on at such a rate as to counterbalance each other, taking the earth as a whole, although there is probably no experi mental or observational demonstration of this hypothesis. It is calculated that at the present time any excess of ammonia or carbonic acid gas would be rapidly absorbed by the ocean water, resulting in the formation of precipitates that would accumulate at the bottom of the sea. In general, living plants absorb carbonic acid gas from the air and give up oxygen to it, while ani mal life absorbs oxygen and gives up carbonic acid gas. It cannot be shown that these two processes exactly counterbalance each other, hut of course the tendency is in that direction. An appreciable portion of the atmospheric gases is held in solution by fresh and salt water, where it is utilized by plants and animals that live in the water.
A large part of the aqueous vapor in the at mosphere is retained for months on the earth's surface as solid snow or ice, while other por tions, forming as rain or dew, either evaporate quickly or accumulate in lakes and oceans and the superficial strata of soil. But so far as is known, there is at the present time no special tendency toward a steady increase of snow on land or rain-water in lakes and oceans, except possibly over the interior of Greenland and in the central portion of the Antarctic Continent, both of which regions are supposed to be in a permanent glacial condition similar to what must at one time have prevailed over large por tions of North America and Europe, and possi bly, in their turn, over other portions of the globe. The steady accumulation of a great mass of snow, and especially its periodical accumula tions and evaporation, must disturb the earth's axis of rotation, producing periodic changes in latitude, such as are even now going on on a very small scale.